I once owned and restored a Canadian Heintzman Bros. player with an Otto
Higel player action.  It had an expression device unlike any I've ever
seen.  A spring-loaded pneumatic was directly slaved to the reservoir, so
they opened and closed in tandem.  Attached to the open end of the
pneumatic was one end of a lever, with the other end connected to the
piano's hammer rail.  As you pedaled lightly, the pneumatic collapsed and
the lever pushed the hammers up towards the strings.

The results were light suction plus short hammer strokes for soft
playing.  As you pedaled harder, about Mezzo Forte, the lever hit a stop.
As the pneumatic further collapsed, the lever pulled the hammer rail
back.  The results were louder, quicker, expression.

Because the design of the Otto Higel was very good, the reservoir and
slave were very touchy.  Expression was the best I've ever seen in an
upright.  It was also possible to lock out the system through a lever in
front of the keys.

I've restored numerous Higels, in Canadian pianos and often in Farrands.
The pillow pouch type are very well designed.

Jeff Davis